Dual battery issues

[ShawnN]

My 99 K3500 came with dual batteries. It had a few wiring problems (loose terminals, blown trailer brake controller, trailer plug full of mud) all of which I have fixed. After fixing those issues it began draining the batteries so I disconnected the aux battery and under-hood light and the problem stopped. I thought I had a door lock relay stuck & causing the problem, which disconnecting the batteries allowed to open. I have been running off the primary battery for several months with no issues so last weekend I reconnected the aux battery as I occasionally do a lot of starting without driving (pulling a trailer around a field to load hay etc on - start, move 20' then turn it off repeatedly for hours). Sure enough, I went to drive it the next day and the batteries were dead. I did reconnect the under-hood light when I put the aux battery back in the loop, but verified it goes out as the hood closes. I took the aux battery out of the loop and again no problems. I had my local shop test the batteries a few months ago and they didn't report any problems. Any ideas what might be causing this?

 

[RichLo]

Check the positive cable for cracked insulation.

Otherwise try a new aux battery. Even if they test good individually if one is 13v and the other is 12.5v they could drain each other trying to equalize.

 

[454cid]

Isn't there some sort of isolator used when running two batteries?

 

[ShawnN]

I've read about an isolator that can be used but also read it's not absolutely necessary. One thing I didn't mention is that the two batteries are rated at different cold cranking amps, not sure if that makes a difference. I am not positive they load tested the battery either, they tend to "forget" things on the list if they're busy - I need to take it back up there and watch while they test it.

 

[ShawnN]

I did a Google search after my last post and found this article: https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel/

I want extra starting capacity, not trying to run an inverter or something and charge via the alternator so according to this I have two problems, different CCA/manufacturer batteries and not properly connecting both to the truck per the article. Oh well, I guess I'll just keep the second battery as a spare and buy two new matching batteries if it ever seems necessary.

Thanks guys for your input.

 

[RichLo]

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[RichLo]

I have one of those in my plow truck, works great!

 

[DallasTahoe]

I have dual batteries, not stock, I use a stinger solenoid to keep the two batteries separated when the truck is off, and once the key is on it opens the circuit and both batteries are joined


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[laidbackbigun]

Yeah different CCA batteries with dtain each other trying to equal out. Hell ive seen same cca batteries different brands do it in the car audio world.

 

[ShawnN]

Yeah different CCA batteries with dtain each other trying to equal out. Hell ive seen same cca batteries different brands do it in the car audio world.

Thank you. It's not worth the expense of two batteries at the present time but now I know what I need to do to make it work.